1. Technical Fields
The present invention concerns an surface acoustic wave device (e.g., surface acoustic wave filter, surface acoustic wave resonator, etc.) as well as a method for its design. In particular, it concerns an surface acoustic wave device that utilizes a single-phase unidirectional transducer, which serves a function of reversing the directionality of an surface acoustic wave as well as a method for its design.
2. Background Technology
In recent years, mobile communication terminals, which are led by mobile phones, have rapidly entrenched themselves. With regard to portability, a compact size and light weight are especially important requirements for such terminals. In order to reduce the size and weight of the terminals, it is essential to reduce the sizes and weights of electronic components included in it, and therefore, surface acoustic wave devices (surface acoustic wave filters), which are advantageous for such attempts to reduce the size and weight, are being used extensively in high-frequency components and intermediate-frequency components of the terminals. The surface acoustic wave device is characterized by a constitution wherein cross-fingered electrodes (interdigital electrodes) are formed on the surface of a piezoelectric substrate as transducers for exciting and receiving surface acoustic waves.
Unidirectional transducers, namely transducers wherein the energy level of surface acoustic waves emitted on one side is relativistically elevated, have been proposed for surface acoustic wave devices for minimizing the bidirectional losses of transducers. The unidirectional transducers can be roughly classified into a triple-phase unidirectional transducer and a single-phase unidirectional transducer. Practical implementations of the former are accompanied by difficulties since an external phase transfer device, etc. are required for generating three phases, whereas the latter is advantageous in that it requires no external phase transfer device, etc.
As far as unidirectional transducers, which can be prepared by photolithographic procedures similar to those for preparing conventional transducers, one which uses the asymmetry of its electrode structure or the internal reflection attributed to a mass-loading effect, one which uses the reflection by a floating electrode, one which utilizes the anisotropy of a substrate (natural single-phase unidirectional transducer), etc. have heretofore been proposed. These single-phase unidirectional transducers are directionalized by concentrating surface acoustic wave energies in the forward direction while phase differentials are being maintained between excitation waves and reflection waves in such a way that they will abide in coinciding phases in the forward direction (default direction or positive direction) and non-coinciding phases in the opposing direction (counterdefault direction or negative direction).
An example of a single-phase unidirectional transducer, which uses a floating electrode, is mentioned in Japanese Patent Application Publication No. Kokai Sho 60[1985]-236312. The floating electrode-internal reflection type unidirectional surface acoustic wave transducer mentioned in said patent publication is obtained by configuring a floating electrode, which is coupled with neither an anode nor a cathode within the gap of a cross-fingered electrode matrix wherein the length between the center of the anode and cathode is .lambda./2 (.lambda. is the wavelength at the central frequency) in such a way that its central position will not coincide with the central position between the anode and cathode. As FIG. 9 indicates, furthermore, Yamanouchi and Furuyashiki ("Acoustic Surface Wave Filters Using Internal Reflection Type of Unidirectional Transducers," Technical Report, Institute of Electronics and Communication Engineers, US84-18, pp. 95-100, 1984) propose the simultaneous insertions of the open floating electrode (62a) and the short-circuited floating electrode (62b) between the positive and negative excitation electrodes (61a) and (61b), respectively, for the purpose of obtaining a more accentuated unidirectional profile. FIG. 11A shows the relationship between the normalized frequency of the unidirectional transducer, the constitution of which is shown in FIG. 9 (frequency normalized to the central frequency) and its conversion loss. The conversion profile shown in FIG. 11A was obtained by using 128.degree. YX-LiNbO.sub.3 as a piezoelectric substrate while the electrode aperture length was being designated at 40 .lambda. (.lambda.: Surface acoustic wave wavelength at the central frequency). The solid curve and dotted curve in FIG. 11A respectively indicate the conversion losses in the forward and backward directions of the unidirectional transducer. The differential between the conversion loss in the forward direction and the conversion loss in the backward direction signifies the magnitude of unidirectionality. In FIG. 11A, the unidirectionality is maximized in the vicinity of the central frequency, based on which a loss--loss filter can be actualized.